The TRR 181 seminar is held by Dr. Valerio Lembo (ISAC - Bologna) on October 16, 3:15 pm in Bundestrr 53, Hamburg, room 22/23.

Concurrent heatwaves and meridional heat transport extremes: an energetic view on the 2021 "Northwestern Pacific” extreme event

Abstract

Concurrent heatwaves are gaining more and more attention within the research community, given their widespread socio-economic impact and their increasing frequency related to climate change. Several studies have been devoted to link examples of such spatially compounded extreme events and the general circulation of the atmosphere with its dominant patterns. We hereby propose an interpretation of the relation between mid-latitudinal atmospheric eddy dynamics and hemispheric land-surface temperature (LST) anomalies, through the mediation of meridional heat transports (MHT). We focus on LST and MHT extremes, and we highlight that warm LST extremes are related to weak MHT extremes in JJA, and that a remarkable example of this conditional occurrence is the concurrent heatwave associated with the regional temperature extremes that affected the Northwestern Pacific coastal region between late June and early July 2021. Looking at wavenumber-decomposed MHTs, we notice that these events feature an often negative (therefore, equatorward) MHT, and that a dominant role is played by the contribution of the k3 wavenumber, typically counteracting the transport by other scales. The phase of this wave is such that ridges are found over the high latitudes of central North America, Scandinavia and Siberia, thus favoring the overall equatorward transport of heat. The relation between extremely warm hemispheric LSTs and extremely weak MHTs is also found in DJF, with k3 equatorward transports again playing a major role, supported by equatorward contributions by k5 and k6. In DJF, though, weak MHT extremes are almost always positive-signed, signaling a balance between different wavenumber contributions. The synoptic patterns associated with co-occurring MHT-LST extremes in DJF suggest that the weak transport enhances a zonal circulation, bringing mild and wet air inland from the oceanic regions (mainly the Northern Atlantic) and discouraging the formation of strong high pressure systems over the cold continental surfaces.